In recent years, environmental, regulatory, and financial factors have prompted new electrical motor designs. In the United States, new laws will soon mandate that commercial and industrial users of A.C. induction motors use only motors which meet specific requirements for energy efficiency. Utilities also provide incentives to commercial and industrial users to encourage the use of these more efficient motors. As a result, induction motors with a high-efficiency design have become more prevalent.
A ratio of inductive reactance (X.sub.L) to resistance (R), known as the X/R ratio, can be found for any motor. This ratio can generally be obtained from the manufacturer, although X/R ratio data is not always distributed with motors. In general, the inventor has found that the design characteristics of the new highly-efficient motors produce X/R ratios greater than those of previous motors.
Because of certain design characteristics of the energy efficient A.C. induction motors, industrial power control circuits may interact with these motors in an undesired manner. In particular, nuisance tripping of instantaneous trip circuit breakers (provided for short-circuit protection) has been a problem during start-up of these motors. It has been determined by the inventor that nuisance tripping is caused by an additive effect of current asymmetries induced during startup of energy efficient A.C. motors. While standard design motors may also draw current asymmetrically, the asymmetric components are much larger in energy efficient motors, and the substantial asymmetries during startup may be misinterpreted by circuit breakers as short circuits between phases producing an unnecessary shutdown.
For safety reasons, it is necessary to maintain exceptional sensitivity and provide an instantaneous response in the circuit breakers associated with these industrial motors. Ground faults and similar problems may present a life-threatening hazard, or may damage costly equipment because of the substantial currents present in the motors and their switchgear. Phase-to-phase short circuits or ground faults may explosively destroy the motors and associated equipment, posing a severe threat to personnel in the area.
Since large startup current asymmetries are inherent in high-efficiency motor designs, and instantaneously responsive circuit breakers are essential to industrial safety, some amount of nuisance tripping has been viewed as unavoidable in industrial applications of high-efficiency motors. In general, motor starter circuits have not been viewed as providing a substantial solution to this problem.
Motor starter circuits that attempt to minimize in-rush current to a polyphase motor have been provided for other reasons, such as to reduce line fluctuations, as disclosed in U.S. Pat. No. 4,628,241 to Bristow. Bristow shows a start-up control method for an induction motor where each phase has a thyristor or triac to control its firing. The first one or two phases are initially fired at a preset angle (35 to 45 degrees) after a zero crossing of the phase voltage. Subsequent firings may be timed for 50-60 degrees after the first firing. The timing angles are successively varied to provide a "soft" start.
Additional motor starters that control phase timing are disclosed in U.S. Pat. No. 4,482,853 to Bhavsar, U.S. Pat. No. 5,206,572 to Farag et al., U.S. Pat. No. 5,168,202 to Bradshaw et al., U.S. Pat. No. 5,140,247 to Verbos, U.S. Pat. No. 4,950,970 to Davis et al., U.S. Pat. No. 4,800,326 to Usworth, U.S. Pat. No. 4,752,725 to Ominato, and U.S. Pat. No. 4,470,001 to Resch et al.
The prior art, however, fails to provide a contactor which can be constructed easily and cost-effectively, yet minimizes nuisance tripping. In particular, as far as the inventor is aware, the prior art does not provide a contactor in which a phase is initially closed in a relationship with a zero crossing based on the characteristics of the particular motor to minimize nuisance tripping.